Conventionally, a method for manufacturing a silicon wafer used as a semiconductor substrate generally includes a crystal growing process for producing a single crystal ingot by a Czochralski (CZ) method, a Floating Zone melting (FZ) method or the like, and a wafer manufacturing process where this single crystal ingot is sliced and at least one main surface thereof is processed into a mirror-like surface. To describe the process more detailedly, wafer manufacturing process comprises: a slicing step of slicing the single crystal ingot to obtain a thin and disk-shaped wafer; a chamfering step of chamfering a peripheral edge portion of the wafer obtained through the slicing step to prevent cracking and chipping of the wafer; a lapping step of flattening this wafer; an etching step that removes machining deformation remaining behind in the so chamfered and lapped wafer; a polishing step of making a mirror surface of the wafer; and a cleaning step of cleaning the polished wafer to remove abrasive slurry or dust particles deposited thereon. The main steps of the wafer manufacturing process are only listed above, and sometimes other steps such as a heat treating step may be added, the above one step may be performed at multiple stages and the step sequence may be changeable.
The polishing step, among the above steps, includes various forms of polishing methods. For example, a mirror-polishing method for a silicon wafer comprises various methods such as a both side polishing method to polish both surfaces of a wafer simultaneously as in the lapping step, a single wafer polishing method to polish a single wafer vacuum-chucked on a plate one by one, and a wax-free polishing method to polish a wafer held by a backing pad and a template without using adhesive such as wax. At present, there is mainly used a polishing method using a wax mount batch type one side polishing apparatus to polish one surfaces of plural wafers adhered onto a plate made of such as glass or ceramics with wax. In this polishing apparatus, the plate on which the wafers are held is placed on a turn table on which a polishing pad is adhered and with a load being applied to an upper top ring the wafer is polished while rotating the turn table and the top ring.
Generally, there are employed as the polishing pads used in the polishing methods polishing pads made of such as a non-woven cloth type or a suede type. A polishing pad of a non-woven cloth type which is fabricated by impregnating a polyester felt (having a texture of a random structure) with polyurethane, has porosity and moderate elasticity and is excellent in a polishing rate and a flatness level, so that polishing with less of a sag is achieved. This kind of polishing pad has been widely employed for stock removal polishing of a silicon wafer.
A polishing pad of a suede type has a base material of polyester felt impregnated with polyurethane, and a foamed layer is formed in the polyurethane, by removing a surface portion of the foamed layer openings being formed thereon (this layer is called a nap layer). This polishing pad is specifically used in final polishing, wherein abrasive slurry held in the foamed layer acts between a work and an inner surface of the foamed layer to thereby perform polishing of the work. Though this type of a polishing pad has been well used in chemical mechanical polishing to obtain a damage-free surface, a peripheral sag is easy to occur during long time polishing. In addition to the above polishing pads, a polishing pad such as a foamed urethane sheet is available.
As a method for fabricating a polishing pad such as a polishing pad of a non-woven cloth type or the like, there is exemplified a process for fabricating a polishing pad with arbitrary characteristic, wherein a polyester felt is impregnated with resin such as polyurethane and the surface thereof is ground with a roll-shaped grindstone attached with super hard abrasive grains thereon (this process is called buffing). A resin material, a content of impregnation thereof and a buffing condition for the surface control compressibility and other properties of the polishing pad of this kind. At present, a polishing pad with compressibility of the order of several % is used and it has been known that a polishing pad with low compressibility contributes to a reduction of the sag in an outer peripheral portion of a wafer.
It is important to keep a constant quality of each of the polished wafers in a polishing step. For the purpose, while stabilization of a polishing pad is important, especially there become issues surface roughness, compressibility, a compressive elasticity modulus, and others.
For example, if the compressibility of the polishing pad is decreased, a problem arises that the influence of precision in a polishing apparatus (a shape of a turn table, surface deflection of the turn table, vibration in processing, etc.) cannot be absorbed to deteriorate flatness of a wafer. Conversely, if the compressibility is excessively large, the amount of surface depression of the polishing pad becomes large to cause a sag in an outer peripheral portion of a wafer.
Also, as for the surface roughness of the polishing pad, in a case of a smoother surface thereof and a higher content of impregnated resin therein, a contact ratio between the polishing pad and an outer peripheral portion of a wafer rises to increase a sag in the outer peripheral portion. Conversely, in a case where the surface roughness of the polishing pad is increased by buffing it roughly, the compressibility itself becomes larger to further increase a sag in the outer peripheral portion.